1. Field of the Invention:
The invention relates to a fluid pressure activated perforating gun for subterranean wells and particularly to an apparatus for assuring that only clean fluid is in contact with the fluid pressure activated apparatus contained in the firing head of the perforating gun.
2. History of the Prior Art:
Perforating guns activated through the application of a fluid pressure differential between the fluid pressure contained within a firing head and the fluid pressure existing in the well annulus adjacent the firing head have been heretofore utilized in well drilling industry. See, for example, the MODEL B FIRING HEAD, Product No. 492-70 sold by BAKER SAND CONTROL DIVISION, BAKER OIL TOOLS, INC. of Houston, Tex. Normally, the perforating gun, together with its fluid pressure activated firing head, is suspended in the well from a packer. A fluid crossover device is incorporated in a depending tubing assembly connected at its upper end to the packer bore and at its lower end to the bore of the fluid pressure activated firing device. Such crossover device effects the diversion of fluid supplied at its upper end bore to the casing annulus.
The fluid pressure activated firing device is then fired by increasing the annulus pressure at the well surface and, through the operation of the crossover device in the tubing assemblage, this produces a fluid pressure increase in the bore of the fluid pressure activated firing device over the annulus pressure existing around the device, thus effecting its actuation and the discharge of the perforating gun. This arrangement has the further advantage of permitting the gun to be discharged when the fluid pressure in the annulus area adjacent the newly formed perforations is in a so-called underbalanced condition; i.e., the fluid pressure is substantially less than the anticipated formation pressure so that an immediate purging flow of formation fluid is produced through the newly formed perforations and into the casing annulus below the packer, then upwardly through the crossover device into the interior of the work string or production string connected to the packer.
Difficulty has been encountered with fluid pressure activated firing devices of the type described above due to the fact that the annulus fluid above the packer may very well be a relatively heavy fluid, such as drilling mud, containing substantial quantities of particulates. It is highly undesirable that this heavy, contaminated fluid be permitted to enter the interior of the fluid pressure activated firing device. Yet, this has been a common occurrence with prior art firing devices.
A similar problem is encountered whenever a subterranean well tool is provided with a chamber filled with a clean fluid which is subject to fluid pressure by the well fluids. For example, it is common to provide chambers filled with a clean fluid for the purpose of trapping a fluid pressure within the chamber which is proportional to the well fluid pressure.